Guide Swift Concurrency

> Guide Skill — This is an expert workflow/pattern guide, not API reference documentation. > Originally from twostraws/Swift-Concurrency-Agent-Skill by Paul Hudson. MIT License.

Swift Concurrency Patterns

Review and write Swift concurrency code for correctness, modern API usage, and adherence to project conventions. Report only genuine problems — do not nitpick or invent issues.

Core Instructions

• Target Swift 6.2 or later with strict concurrency checking.
• Prefer structured concurrency (task groups) over unstructured (Task {}).
• Prefer Swift concurrency over GCD for new code. GCD is still acceptable in low-level code, framework interop, or performance-critical synchronous work.
• Do not suggest @unchecked Sendable to fix compiler errors — prefer actors, value types, or sending parameters.

Review Process

1. Scan for known-dangerous patterns using references/hotspots.md.
2. Check for Swift 6.2 concurrency behavior using references/new-features.md.
3. Validate actor usage for reentrancy and isolation using references/actors.md.
4. Ensure structured concurrency is preferred using references/structured.md.
5. Check unstructured task usage using references/unstructured.md.
6. Verify cancellation handling using references/cancellation.md.
7. Validate async stream and continuation usage using references/async-streams.md.
8. Check bridging code between sync and async using references/bridging.md.
9. Review legacy concurrency migrations using references/interop.md.
10. Cross-check against common failure modes using references/bug-patterns.md.
11. If strict-concurrency errors exist, map diagnostics to fixes using references/diagnostics.md.
12. If reviewing tests, check async test patterns using references/testing.md.

If doing partial work, load only the relevant reference files.

References

| Topic | Reference | |-------|-----------| | Grep targets for code review — known-dangerous patterns | references/hotspots.md | | Swift 6.2: default isolation, @concurrent, Task.immediate, isolated deinit | references/new-features.md | | Actor reentrancy, global actor inference, isolation patterns | references/actors.md | | Task groups, async let, withDiscardingTaskGroup, concurrency limits | references/structured.md | | Task vs Task.detached, when Task {} is a code smell | references/unstructured.md | | Cooperative cancellation, withTaskCancellationHandler, broken patterns | references/cancellation.md | | AsyncStream.makeStream(of:), continuation lifecycle, back-pressure | references/async-streams.md | | Checked continuations, wrapping delegates, @unchecked Sendable | references/bridging.md | | GCD migration, Combine to AsyncSequence, completion handlers, locks | references/interop.md | | Common concurrency failure modes LLMs produce and their fixes | references/bug-patterns.md | | Strict-concurrency compiler errors mapped to likely fixes | references/diagnostics.md | | Async test patterns, race detection, avoiding timing-based tests | references/testing.md |

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: vabole
• Source: vabole/apple-skills
• License: MIT

Install and usage instructions live in the source repository linked above.